The relatively high price of natural gas has recently resulted in the closure of a few inefficient ammonia plants in the US. To make matters worse, natural gas prices are expected to remain high in the foreseeable future. Thus, for ammonia plants to become and remain profitable, it will be imperative to increase revenue and reduce operating costs. Viewed from another perspective, plants will have to find ways to increase production and reduce their energy consumption per ton of ammonia produced.
In a typical prior art plant configuration as depicted in Prior art FIG. 1, natural gas is processed through primary and secondary reformers, and the reformed gas stream is then subjected to a shift conversion after excess heat has been recovered from the reformed gas stream. In a further step, acid gas (here: carbon dioxide) is removed, and residual carbon monoxide (CO) and carbon dioxide (CO2) are almost completely eliminated in a methanator. The resulting gas stream is then passed into the synthesis loop for production of ammonia, wherein the nitrogen is typically provided from process air that is fed into the secondary reformer.
Production capacity can be increased by removing excess nitrogen from the syngas stream by combination of gas expansion, autorefrigeration, and cryogenic separation as for example described in U.S. Pat. No. 3,442,613 to Grotz. While Grotz's configuration is relatively effective in some ammonia production plants, retrofitting of existing plants is relatively problematic since the expansion step requires considerable increase in front-end pressure in a ammonia synthesis plant (and thereby typically necessitating retrofits/rebuilds to accommodate for the increased pressure).
To circumvent at least some of the problems associated with retrofitting existing ammonia synthesis production plants, Bhakta describes in U.S. Pat. No. 5,935,544 a configuration in which a purifier process has been applied to a low methane content syngas with moderate N2 excess, wherein an external refrigeration step produces a syngas with sufficiently low inert content to significantly increase synthesis capacity or decrease in synthesis loop pressure. Although Bhakta's configuration overcomes in many cases the need for retrofitting existing plants, Bhakta's configuration is typically limited to relatively narrow process parameters in order to provide satisfactory improvements in synthesis capacity.
Although there are various configurations and processes for improvements of ammonia synthesis are known in the art, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need to provide improved methods and configurations for improved of ammonia synthesis.